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That is be true, John, but there are certainly examples where "further" cannot be replaced by "farther". "Further" may also mean "additional" while "farther" cannot."Here are further examples of this" cannot be replaced by "Here are farther examples of this." and you have never heard "farthermore", have you?:)

I will always use "further". Maybe it's because I'm a purist, and Australian, but I do not use "farther" in any context.See, if I say "It is further away", "futher" is correct because it is being used in the time context, even though distance is inferred...